He told the Radio Times that it suited many climate change sceptics to "be that way" because it made their own life easier.

Asked whether he was hopeful or despairing about the future of the planet, he said: "I'm on the pessimistic side. I don't think there's any question that things are going to get worse."

Sir David, whose wife of almost 50 years, Jane, died of a brain haemorrhage in 1997, said that absorbing himself in the natural world had helped him cope with grief.

He said: "In moments of grief - deep grief - the only consolation you can find is in the natural world.

"People write to me and tell me this. People of great distinction have written and said, 'When so-and-so died, the only thing that made life tolerable was to watch programmes on plants and animals'."

The wildlife presenter said that his brother Richard, who directed the films Cry Freedom and Gandhi and starred in The Great Escape and Jurassic Park, was "not well".

He said: "I don't fear death, but I fear suffering, of course. Who wouldn't?"

He said work, rather than "filling in time, like playing golf," gave him a reason to get up at four o'clock in the morning. "I am blissfully blessed that people want me to do something, so why should I say no?"